1. Field of the Invention
The present invention relates generally to insulated supports for transmission lines, and more particularly to an insulated mounting support for a ladder-line transmission line.
2. Description of the Prior Art
In high frequency radio systems (e.g., 3-30 MHz), coax cable or "ladder-lines" are used to transfer power from a transmitter to an antenna's radiating element. Commercially available "ladder-line" transmission line provides a balanced feed system, is the least lossy transmission line in terms of power, and is the least expensive of all power transfer alternatives. The conventional "ladder-line" consists of two spaced apart parallel conductors that are separated and encased by a dielectric material. The dielectric material is provided with a number of spaced apart cutouts along the transmission line, the size, shape and spacing of which determines the characteristic impedance of the transmission line.
For example, as shown in FIG. 1, a conventional 450-ohm "ladder line" transmission line 10 has two spaced apart parallel conducting wires 11 and 12 encased in and separated by a dielectric material 14. Dielectric material 14 is provided with a plurality of spaced apart rectangularly shaped cutouts 16 that yield 450-ohm characteristic impedance. Wires 11 and 12 are exposed and extend from transmission line 10 at one end thereof. This end of transmission line 10 is commonly referred to as the feedpoint. In the prior art, attachment of the feedpoint of transmission line 10 to an antenna radiator 100 is as follows. The exposed portion of wires 11 and 12 are fed through holes 201 and 202, respectively, of an insulator body 200. Each of wires 11 and 12 is then twisted with respective radiator wires 101 and 102 at respective twist points 151 and 152, and soldered in place.
The problem with this system is that tension on transmission line 10 places a strain on wires 11 and 12 at insulator body 200. Over a period of time, movement of transmission line 10 causes fatigue in and eventual breakage of wires 11 and 12 thereby requiring repair. Such repair of the broken wires may be difficult if insulator body 200 is maintained in a limited access location, e.g., a tall antenna tower.